This invention relates to an optical recording medium such as an optical disk on which information is recorded and from which the information is reproduced by irradiating a laser beam thereto, and more particularly to an optical recording medium including an optical recording layer having a substantially flat thickness and a substantially flat surface over the layer.
An optical disk has been conventionally known as a recording medium. In such a conventional optical disk, information is recorded on an optical recording layer formed in the optical disk by locally irradiating a light beam to the optical recording layer to heat the irradiated portion of the layer and change a physical property such as a magnetization direction, a phase state, an absorption spectrum or the like in the irradiated portion of the layer, and the information is reproduced from the optical disk by detecting the rotation of the polarization plane of a light beam reflected from the optical recording layer (that is, Kerr effect), the rotation of the polarization plane of the light beam passing through the optical recording layer (Faraday effect), or the change of intensity of the reflected light beam. In this case, a magnetooptic material, a phase-change material, an organic pigment, a pit-formable material or the like is used for the optical recording layer of the optical disk. Further, the optical disk comprises a base plate having projections or grooves (that is, irregularity) formed spirally or coaxially on the surface thereof which are used for defining tracks, an interference layer or protection layer, the optical recording layer as described above and a protection layer which are laminated in this order. Each of those layers of the conventional optical disk has the sectional profile corresponding to the irregularity of the surface of the base plate which is caused by the projections or grooves.
FIG. 1 is a cross sectional view of a conventional optical disk in which a magnetooptic material is used for the optical recording layer. The conventional optical disk comprises a disk base plate 12' having projections 12a' (irregularity) formed spirally or coaxially on the surface thereof, an interference layer 14', a magnetooptic recording layer 16' and a protection layer 18'. The interference layer 14' and the protection layer 18' are formed by the thin film forming method such as a sputtering method, a vacuum deposition method or the like. As shown in FIG. 1, the magnetooptic recording layer 16' is formed on the base plate such that the layer 16' has substantially the same sectional profile as that of the irregularity of the surface of the base plate, that is, the recording layer itself has an uneven surface, so that it is difficult to obtain perpendicular magnetic anisotropy uniformly over the recording layer because the thickness of the recording layer is not constant over the layer and the direction of the magnetization is different between stepped portions and flat portions of the surface of the recording layer. This causes a recording characteristic of the optical disk to be unstable. As a result, the shape of bits (elements constituting information in the recording layer, for example, pits) is ununiform and a signal-to-noise ratio (S/N) is reduced.
Similarly in the optical disk as described above, a conventional optical disk, in which a phase-change material is used for an optical recording layer and the surface of the recording layer is rough, also has disadvantages that the thickness of the recording layer is not constant over the layer and crystallization is deteriorated between stepped portions and flat portions of the surface of the recording layer, so that an uniform reflectivity or transmissivity for an incident light is not obtained. This causes noise components to be increased and the recording characteristic of the optical disk to be degraded. Moreover, the recording layer is liable to be oxidized and deteriorated at the stepped portions, and therefore, the optical disk having such an optical recording layer has a low reliability for employment for a long time.
Still further, in the conventional optical disk as described above, only the optical recording layer is used for recording and reproducing information, so that the memory capacity of the optical disk is limited. Such a conventional optical disk can not satisfy a requirement for increasing the memory capacity of the optical disk, which has been made larger in the art as the demand for optical disks is increased.
Still further, in the conventional optical layer, thickness of the protection layer is set to be about .lambda./4 where .lambda. is a wavelength of light in a medium and the thickness of the optical recording layer is set to a predetermined value so that the reflectivity for the light is substantially minimum in order to enhance a Kerr effect, so that the intensity of a diffracted light from the reflection layer for a tracking operation is reduced. This causes a tracking servo for the optical disk to be unstable and a stable reproduction is not performed.